Separation of oxygen from air by liquefaction



Oct. 17, 1944. A. c. BROWN 2,360,458

SEPARATION OF OXYGEN FROM AIR BY LIQUEFACTION Filed May 29. 1939 2Sheets-Sheet 2 Patented Oct. 17, 1944 SEPARATION F OXYGEN FROM AIR BY iLIQUEFACTION Andrew C. Brown, Wilmette, lll., assigner to CompressedIndustrial Gases, Inc., a corporation of Delaware Application May 29,1939, Serial No. 276,325

Claims. I (Cl. 62-122) One of the objects of the invention is the proivision oi a new and improved pressure operated mechanism forautomatically maintaining proper phase equilibrium or proper balancewithin the 4 system.

Another object of the invention is the provision of new and improvedmeans for automatically controlling the operation of the rectifyingapparatus by maintaining a predetermined height ci the crude oxygen bathin the high pressure rectiiying column.

Another object of the invention is the provision oi new and improvedpressure operated mechanism for controlling the supply of refluxdelivered to thelow pressure column, the mechanism being so constructedthat none of the moving parts will be subjected to the low temperatureof liquid air or oxygen.

A. further object of the invention is the provision of a new andimproved mechanism for automatically maintaining the entire system invproper equilibrium so that the production of oxygen of a high degree oipurity may be ob` tair'ied in a continuous or uninterrupted process.

Another object of the invention is the provision of new and improvedmechanism for removing carbon dioxide from the crude oxygen during theoperation ofthe Oxygen producing apparatus. e i

A still further object oi the invention is the provision of a new andimproved method of producing oxygen in a continuous process and on aneconomical'and productive scale.

A.further object of the invention is the provision of a new and improvedapparatus for producing oxygen gas and for automatically controlling theoperation of the apparatus, that is simple in construction, edicient inoperation, and that is composed of a minimum number of moving parts. n y

. Other and further objects and advantages of the invention will appearfrom the following description,`taken in connection with theaccompanying drawings, in whichs Fig. l is a side elevation of a doublerectifying column device showing theparts diagrammatica ly; viFig. 2 isa similar view oi' a single column de- Fig. 3 is a vertical section ofthe expansion valve and the control therefor; and

Fig. 4 is a modified form of expansion valve and control therefor.

In the production of oxygen gas and in the control oi' the reiiux andphase equilibrium, it is the common practice to control the operation ofthe apparatus, and especially the operation of the expansion valves, bymanually operated means. For economical operation, it is necessary thatthe amount of @uid ilowing through the expansion valve shall be suchthat at the discharge into the column, the liquid shall be in phaseequilibrium with the surrounding fluid in the column.

in the production of oxygen gas and in the control of the reflux andphase equilibrium in the columns, it is the common practice to controlthe operation of the system by manually operated expansion valves. Suchan arrangement is objectionable because of the close attention necessaryto the operation of the system. Where the valve is manually controlled,solidied CO2 and other particles whose criticals are above that ofoxygen and nitrogen will restrict and even clog the passage therebynecessitating the adjustment of the valve to restore the normaloperation of the column. But where automatically controlled, the valveis automatically opened when clogged for permitting the obstruction topass through' the same.

The present invention seeks to provide novel means for eiecting thecontrol of the system by automatically operated mechanism which in turnis controlled indirectly by the level of the crude oxygen bath Withinthe rectifying column.

Referring now to Fig. 1 of the drawings, which is a diagrammatic viewshowing a double column rectiiier, the reference character l0 designatesthe rectifier column which comprises the high pressure section Il andthe lower pressure section l2 vertically above the high pressuresection. The two sections are separated by the partition i4 whichconstitutes the flue sheet of a condenser il. The column is of awell-knowntype, except for the condenser I1, and each section isprovided with evaporating trays, those of the lower section being showndiagrammatically at I5 and i 8. These trays are of the conventionalconstruction and need not be described in detail.

The condenser Il comprises the ue sheet i4, a header I9 and a pluralityof tubes or ilues 20 between the flue sheet I4 and header I9. These uesor tubes place the upper section of the column in communication with theheader i9. The

header I9 and ues or tubes 20 contain almost pure liquid oxygencondensed in the upper section of the column, as will presently bedescribed. v

The upper section I2 of the column is enlarged at its lower end as at I8to extend below the ue sheetk I44 and condenser I1 and is attached tothe lower section II below its upper end to form a reservoir 2I forcontaining liquid nitrogen precipitated by the condenser I1 in the lowersection of the column. In other words, the temperature of the oxygen inthe condenser, being below the critical temperature of nitrogen at thepressure obtaining in the lower section or high pressure section of thecolumn, the nitrogen constituent of the iluid evaporated from the bathcollected in the lower portion of the column will pass upwardly beyondthe trays I and I6 and be condensed by the condenser I1 and collected inthe reservoir 2 I, while the oxygen constituent of this fluid will becondensed by the trays I5 and I6 and fall back into the liquid in thebottom of the column thus adding to the richness of the oxygen upper endof the upper section of the column by means of a passage 24 which, whenthe system is operating, conducts nitrogen gas from the upper end of thecolumn to the lower end of the container 23. This gas'passes through thecontainer and is discharged through the pipe 25.

The air, after it has been compressed to say around 700 or 750 lbs., isconducted through the heat exchange device 22 through a pipe 26 whichtakes a sinuous path through the heat exchange device for increasing itsradiating surface and has its temperature greatly reduced by the coldnitrogen gas ilowing countercurrent through the container and by therened oxygen which isA conducted counter to the flow of air from thereservoir I9 by a pipe 21.

The pipe 21 is in the form of a coil extending about the pipe 26 in theheat exchange device for assisting in reducing the temperature of theincoming air. As a result', the air has its temperature greatly reducedso that it is liquefied by the time it passes from the heat4 exchangedevice. This-liquid is conducted from the heat exchange device throughthe pipe 26 to a heat exchange member in the form of a heating orboiling coil 28 which, inthe form of the construction shown, is a coilthat is normally immersed in the bath of crude oxygen 29 in the bottomof the rectifying column,

The temperature of the liquid in the coil 28 is above that of the crudeoxygen of the bath 29 surrounding it and as a result, the liquid in thebath will be evaporated and will rise upward in the high pressure columnII where the oxygen is condensed and falls back into the bath and thenitrogen is condensed and collected in a separate chamber or reservoir.as is well lmown in rectifying columns.

A suitable expansion valve 3I is provided forvv discharging the liquidair contained in the heat exchange 23 into the high pressure section IIof the rectifying column. As shown, the pipe 32 is in communication withthe' coil 28 and the valve 3I. The iluid passing the expansion valve 3|is discharged through the nozzle 33 in the lower portion of said column.A hand operated valve 34 is provided for manually controlling the now ofliquid through the conduit 32. In the normal operation of the device,the valve 34 is moved to full open position and left that way.

Suitable means are provided for automatIcally controlling the ilow ofthe liquid air through the expansion valve 3|. Any suitable means may beemployed for this purpose. In the form of the construction shown, whichis by way of example only, a pressure controlled valve is employed forthis purpose. As shown in Fig. 3, this valve comprises a body portion 35having a hollow head 36 on its outer end. The body portion 35 isprovided with an axial bore 31 through which a valve plunger 38 looselyextends. The inner end o! the body portion is provided with a plug 39having a valve seat 4I thereon which is in alinement with the valveplunger 38 and is adapted to be engaged by said plunger for closing thepassage 31 therethrough. The pipe 32 is connected to the body portion 35of the valve and is adapted to deliver the liquid air to the valve.

The amount of fluid passing through the valve will be determined by theposition of the plunger 38. 'I'he operation of the plunger 38 iscontrolled by a diaphragm 42 which is clamped between the two sections43 Aand 44 ofthe head 36. The plunger 38 extends through the diaphragm42 and is clamped to the diaphragm lby suitable nuts and washers 45threaded on the outer end of the plunger 38 at each side of thediaphragm 42.

'Ihe extent to which the valve may be opened, that is, the range ofmovement of the plunger 38, may be adjusted as desired. This isaccomplished by adjustably connecting the head portion 36 to the bodyportion 35 of the valve 3I. In the form of the construction shown,` thisis accomplished by threading the outer end oi the body portion 35 into aneck 40 of the head portion 36. The neck is provided with a counterborefor receiving packing held byv a gland 60 for forming a gas and liquidtight joint at this point. When it is desired that the Valve shall becapable of `being opened wider, the 4head is partially unscrewed fromthe body portion which will adjust thediaphragm 42 farther.' from thevalve seat 4I thereby permitting the valve to open wider.

The operation of the diaphragm 42, which vcontrols the movement of thevalveplunger 3l,

is controlled by a pressure sensitive mechanism 46. This mechanismcomprises a container 41 having a conduit 48 connecting the lowerportion of the interior thereof with the high pressure section of thecolumnv beneath the surface of the crude oxygenba'th. A second p ipe orcon"- duit 49 places the upper portion of the container 41 incommunication with the column abovev the --level of the liquid, thearrangement being such that the liquid in the column may pass intojthecontainer and be maintained at the same level as the crude oxygen bath.It is so mounted, however, 'that during the normal operation of thesystem, the level of the liquid in the container 41 will be intermediatebetween the top and lower portions thereof.

The chamber 41 has mounted therein a coil of pipe one end of whichextends beyond the container'and is provided with a valve 62. forclosing the pipe. 'I'he other end of the coil 5| extends through thewall of the container and is in communication with the hollow head 38outwardly of the diaphragm 42. The coil 5i, including the externalportion thereof, is illled with a suitable gas such as hydrogen orhelium that will not liquefy under the temperatures and pressures usedin the rectifying column. By means of this arrangement, the pressure.within the air duct 32 tending tov force the diaphragm 42 and with itthe valve plunger 26 outwardly, will be ccunterbalanced by the pressureof the gas within the pressure control device 5|. It will thus be seenthat when, for any reason, the liquid in the bath'29- becomes colderthan it should be, the pressure within the conduit 32 will move thediaphragm 42 outwardly, due to the diminished pressure of the gas withinthe coil 5|, for opening the valve wider and permitting a greater flowof liquid air through the pipe 26 which, in turn, will warm the crudeoxygen and cause the evaporation of the liquid in the bath 29. Shouldsolidified COa or other solidi- :lied constituents of the air restrictthe valve opening, the pressure in the air duct or conduit .32 willincrease and this increased pressurefon the inner side of the diaphragm42 will move the plunger or valve stem 39 outwardly thus opening thevalve and permitting the obstruction to pass into the column.

If the level of the liquid in the bath 29' rises, more of the coil 5|will be immersed thus cooling the gas within the pressure controlleddevice causing it to contract thereby opening the valve 3| wider andpermitting more compressed air to ow through the tube. On the otherhand,the

lowering of the level of the liquid in the bath 29 will lower the levelof the liquid within which the coil 5| is immersed thereby causing thegas to expand by added heat and moving the valve 3| toward closedposition.

The column I0, heat exchanger 22, filter 53, and control mechanism 46are preferably provided With suitable heat insulating materialv 16. Thecontrol mechanism 46 is without the col- -umn and between the column andthe exterior of the insulation. It is positioned without the columnbecause there is not suilicient heat differential between the liquidcrude oxygen and its vapor to cause it to properly operate. It is placedwithin the insulation but near the surface so that it will besuillciently sensitive to function properly and yet not absorb so muchheat as to .be uneconomical.

Alow pressure section l2 of the column, as will now be described.

The liquid air, in passing through the coil 29, will cause the crudeoxygen in the bath 29 to boil and as the vapor rises, the oxygen willbecondensed in the evaporating trays I5 and I6 will also be washed by theliquid air discharged by the nozzle 33. The nitrogen gas will rise inthe lower chamber and be condensed in the condenser |1 by the liquidoxygen contained therein and will be collected in the receptacle 2|. aswill presently appear.

This crude oxygen, less carbon dioxide, which is solidified in crystalswithin the liquid, will be conveyed through a pipe 52 to a filter 53. 'I'he pipe enters the lower end which will contain more orl of the lterchamber l54 and terminates adjacent the upper end thereof and thechamber is lled around the pipe with suitable filter material 55. A pipe56 leads from the upper end of the cham- Iber 54 into the heat exchanger22 for conducting the carbon dioxide gas through the heat exchanger anddischarges the same through a valve 5l in the upper end of the pipe 56that extends beyond the heat exchange. The filtered crude oxygen isconducted from the lower part of the chamber 54 through a pipe 56through an expansion valve 59 to the nozzle 6i in the upper section ofthe column.

The expansion valve 59 is of the same'type as that shown in Fig. 3 andits description need not be repeated.

The regulating pressure for counteracting the pressure in the pipe 58 onthe diaphragm 42 of this valve is obtained from the compressed air pipe26. A branch pipe 62 conducts compressed air to the head 36 of the valve59. The pipe 62 is provided with a pressure regulator 64 which controlsthe pressure on the outer side ofthe diaphragm of the valve 59. Thispressure remains constant and is comparatively low as for instance aboutlbs. Since the pressure regulator 64 is of the conventional type, itsdescription is not thought necessary.

The crude 4oxygen from the bath 29 andthe nitrogen from the reservoir 2|are forced through the valves 36 and 69 by the pressure developed by thevapor from the boiling crude oxygen in the bath 29 and this pressuredepends on the operation of the valve 3|. For instance, if the amount ofliquid air owing through the coil 28 is small, the boiling of the crudeoxygen will likewise be small and the pressure in the section of thecolumn I0 will be reduced and this will result in the pressure inthepipes 1| and 12 partially or entirely closing the valves 36 and 69.

On the other hand, if the valve 3| is openedito admit 'the passage ofmore liquid air, this air passing through the bath 29 will evaporatemore crude oxygen,and result in greater pressure in the section li. Itwill thus be seen that the automatic valves 36 and 69 depend, fortheiroperation, 'on the proper operation of the automatic valve 3|, therebymaintaining a proper balance in the operative conditions obtaining inthe two'sections of the column.

The crude oxygen passing through the pipe 58 is discharged through thenozzle 6| for reuxing the liquid. 'I'his iiuid will pass over theevaporating pans 65 andl 66 which are similar to the pans |5-and I6 forcondensing the oxygen. v'l'he condensed liquid falls on the condenserand is collected yin the header or reservoir I9 as oxygen comparativelyfree from other gases.

The oxygen liquid is removed through the pipe 21 which conducts the samethrough the-heat exchanger 22 where the oxygen absorbs heat .from theincoming compressed air and is convertedlto a'gas at a comparatively lowpressure. The upper 'end of the pipe r21 extends outwardly through thewall of the heatexchanger 22 as shown at 61. AFrom thence the oxygen, inthe form of a gas, is delivered to compressors or containers, not

The Acondenser |1 is similarI to the iiash type of steam boilers andfunctions `very eiectively to condense the nitrogen gas in the highpressure section of the rectifying column. It will be noted that thepure oxygenis removed from the bottom of the reservoir vand as aconsequence;4 vany amount of oxygen removed resultsln a movement of theentire mass of liquid oxygen in the condenser whereby not only freshliquid oxygen is continually supplied to the reservoir, but the liquidis brought more eiectively into contact with the walls o1' the condenserand as a result. the condenser functions very efliciently for condensingthe nitrogen in the lower section of the column. The liquid nitrogenthat collects in the receptacle 2| is utilized for washing down orcondensing the oxygen which rises from the condenser an-d is evaporatedfrom said reflux.

The temperature within the lower pressure section of the column isregulated by the automatic controls and is maintained between thecriticals of oxygen and nitrogen at the pressure obtaining therein sothat the oxygen will be condensed and the nitrogen collected as a gas.in the upper portion of the column. As shown, a. pipe 68 conducts theliquid nitrogen from the reservoir 2| through an expansion valve 69which is substantially the same as that shown in Fig. 3 and itsdescription need not be repeated. The pressure on the outer side of thediaphragm of this valve is obtained through the pipe 62 and pressureregulator 84 as was the valve 59 previously described.

The pipe 62 is provided with branches 1| and 12 which conduct thecompressed air to the valves 89 and 59, respectively. The liquidnitrogen is discharged at the upper end of the column by a nozzle 13 andwill condense the oxygen in the vapors rising in the upper section ofthe column and will in turn be vaporized. 'Ihe nitrogen vapor will becarried oi by the pipe 24 and delivered to the heat exchange 23 aspreviously described.

In order that the system shall operate economically, the nozzles for theexpansion valves are so located within the rectifying column that theiluid discharged therefrom will be in phase equilibrium with thesurrounding fluid.

Instead of employing the pressure of the compressed air in the pipes 26and 62, the expansion valve shown in Fig. 4 may be employed. This valveis substantially like the valve shown in Fig. 3 except that thediaphragm 42a, which operates the plunger 39a, is held againstv outwardpressure by an adjusting screw 14 which engages a follower 15 betweenwhich and a washer 16 is interposed a compression spring 11. Thepressure applied to the diaphragm 42a may be adjusted to any desiredamount by the screw 14.

It will thus be seen that even though there is not quite 12 C. diierencebetween the boiling points of oxygen (-182.5 (-194" C.) at ordinarypressures, the automatic controls may be so adjusted that both sectionsof the column maybe maintained in phase equilibrium and the oxygenseparated from the nitrogen by rectification.

'Ihe form of the construction shown in Fig. 2 differs from that justdescribed in that it is a single column apparatus. In this form ofconstruction, the upper end 18 of thecolumn 19 constitutes a heatexchanger and the compressed air enters the section 18 through a pipe 8|which is formed into a coil within the section 18 as shown in Fig. 2 ofthe drawings. From the heat exchanger 18, the compressedr air isconducted by a continuationof the pipe 8| to the lower portion of thecolumn which, after the device is in operation, has, in the bottom`thereof, an oxygen bath 82. The pipe-8| is in communication with a heatexchange coil-88 immersed in this bath. From the coil 83, the liquid airis'conducted through rst the manually operated valve 84 and C.) andnitrogen` then the automatic pressure regulated valve 8l and isdischarged in the upper portion of the lower section 86 of the column.The valve 8l is controlled by a mechanism 81 similar to the device shownat 46 and 41 in Fig. 1. In other Words, the mechanism 81 is connected tothe lower section of the column above the level of the liquid by a pipe9| and below it by a pipe 92 so that the level of the liquid in thedevice 81 will be the same as that in the column. The raising orlowering of the liquid in the device 81 will operate the diaphragm inthe same man,- ner as already described.

The liquid oxygen is removed from the bath 82 by a pipe 93 whichconducts the liquid oxygen through the heat exchanger 18 where itabsorbs heat from the compressed air coming in through the pipe 8|. Thepipes 8| and 93 are, in practice, in close contact so that there will bethe proper interchange of heat. 'I'he oxygen. which, due to theabsorption of heat, will be in the form of gas, may be delivered throughthe valve 94.

The temperature of the lower portion of the column will be maintainedabove the critical temperature of nitrogen gas by the automatic valve 85so that the nitrogen constituent of the air will pass as a gas at lowtemperature through the heat exchanger 18 for lowering the temperatureof the compressed air entering the system through the conduit or pipe8|. The nitrogen gas escapes through a conduit into the air or into acontainer, if it is desired to collect the nitrogen.

In the form of the device shown in Fig. '1, the nitrogen passing alongvthe pipe 25 may be, and preferably is, conducted to suitable retainersor'collectors and likewise the carbon dioxide gas passing through thevalve 51 may be discharged into the air or, if desired, it may beconducted to suitable receivers or suitable storage tanks.

While the apparatus is disclosed as being adapted for separating oxygen,nitrogen and carbon dioxide from the air, it is understood that theinvention is not so limited as it may be used as an apparatus forseparating the other constituents of the air, or it may be used inseparating and rectifying elements or compounds of other mixtures thatmay be reduced to gaseous form.

It willthus be seen that the control mechanism for the expansion valvesdepends for its operation, eitherdirectly orindirectly,on the operationof the mechanism for operating the expansion valve 3|. None of theoperating mechanism, or at least the moving parts, is so exposed to lowtemperatures as to affect its operation. This is oonsidered an importantfeature of the invention.

It is thought from the foregoing, taken in connection with theaccompanying drawings. that the construction and operation of my devicewillbe apparent to those skilled -in the art an'd 'that'cha-nges insize, shape, proportion and details of construction may be made withoutdeparting from the spirit and scope of the appended claims.

I claim as my invention: 'j'

l. In a system for producing oxygen from air, a rectifying columnadapted to contain a bath of crude oxygen in its lower end, heatinterchange means within said bath, a heat exchange device, means forconducting nitrogen uid at low temperature to and from said device,means for conducting air under high pressure through said device forliquefying the air and for delivering the liquid air to said heatinterchange means, means for conducting the compressed air fluid to areduction valve for discharging said fluid into said column above saidbath, and means controlled by the height of the liquid in said bath forcontrolling the operation of said valve.

2. A method of separating oxygen from air that comprises compressing theair, cooling the air continuously by passing a stream of cooledcompressed air through a bath of crude oxygen for evaporating nitrogengas therefrom in a continuous process, condensing the nitrogen gas, andcontrolling the amount of air flowing in said stream by pressureresponsive means controlled by the height of the level of the liquidbath.

3. An expansion valve for use in rectifying columns comprising a head, adiaphragm in said head, a valve member secured to said diaphragm, achamber, means .for placing the chamber in communication with the lowerportion of a rectifying column, and a coil containing a highlyexpansible fluid in said chamber, said coil having one end incommunication with said head outwardly of said diaphragm.

4. In a system for producing oxygen from air, a rectifying column havinga high pressure portion and a lower pressure portion, the lower end ofthe high pressure portion of said column being adapted to contain acrude oxygen bath, means for conducting compressed air through said bathand for discharging the same into said high pressure portion above saidbath, an expansion valve for controlling the discharge through saidmeans, pressure responsive means exterior of said bath but aiected bythe temperature thereof for automatically controlling the operation ofsaid valve, conduit means for conducting liquid from said bath to thelow pressure portion of said column, and means for automaticallycontrolling the amount of liquid flowing through said conduit means.

5. In an apparatus for separating oxygen and nitrogen from the air, arectifying column, a condenser dividing the column into an upper lowpressure section and a lower high pressure section, a heat exchangerdevice, means for conducting compressed air through said exchangerdevice to said high pressure section, said condenser comprising apartition wall having apertures therein, tubes secured in said aperturesand extending downwardly therefrom, a chamber in communication with thelower ends of said tubes,

lil

said column being enlarged about said condenser and provided with anannular reservoir below said condenser, means, including said chamber,for condensing nitrogen gas in said high pressure portion of said columnand for collecting the condensed nitrogen gas in said reservoir, meansfor liquefying oxygen and collecting the same in said condenser forassisting in condensing ni' trogen gas in the high pressure portion ofsaid column, means for conducting liquid nitrogen from the high pressure`to the low pressure section of said column and for spraying the sameinto the upper portion of said column, and means for conducting liqueedoxygen from the lower portion of said condenser through said heatexchanger device for cooling the incoming compressed air.

6. In a ysystem for producing oxygen from air, a rectifying columnadapted to contain a bath of crude oxygen in its lower end, a heatexchange coil in said bath for evaporating the liquid of said bath, aconduit for conducting compressed air to said coil, a conduit forconducting liqueed air from said coil to a point above said bath withinsaid column, means anterior of said bath for cooling said air withinsaid first-named conduit, an expansion valve in said last-named conduitabove said bath, and temperature responsive means including mechanismoperated by differential pressure for automatically controlling theamount of fluid discharged through said valve into said column abovesaid bath.

7. In a system for producing oxygen from air, a rectifying column,insulation extending about said column, said column adapted to contain abath of crude oxygen in its lower end, a heat exchange coil in said bathfor evaporating the liquid of said bath, a conduit for conductingcompressed air to said coil, means anterior of said coil for coolingsaid air, and means affected by the level of the bath in said column forcontrolling the operation of said column, said means comprising areceptacle in communication with the lower portion of said column bothabove and below the level of the liquid in said column for receiving aliquid from said bath said receptacle being within said insulationoutside of said column, an expansion valve above the liquid level withinsaid column, adapted to discharge liquid from said coil into said columnabove the level of said bath, and heat responsive means affected by thetemperature of the liquid within said receptacle for controlling theoperation of said valve.

8. In a system for producing oxygen from air, a rectifying column havinga high pressure portion and a low pressure portion, a crude oxygen bathin the lower end of the high pressure portion, means for conductingcompressed air through said bath and for discharging the same into saidhigh pressure portion above said bath, a passage for conducting liquidfrom the high to the low pressure side of said column and fordischarging the same into said low pressure side of the column above thelower portion thereof, an expansion valve in said means for controllingthe discharge through said means, and differential pressure responsivemeans for controlling the operation of said valve.

9i' In an apparatus for separating the component parts of gas mixture byrectification comprising'a rectifying column having a high pressuresection adapted to have a bath of liqueed gas in its lower portion, aheating coil within said bath, a heat exchanger exterior of said columnand including a conduit for conducting said` gas mixture in compressedcondition through said heat exchanger to said heating coil, an expansionvalve, a conduit for conducting the compressed gas from said heatingcoil to said valve for discharging the same into said column above saidbath for lowering the temperature of said bath below the criticaltemperatures of certain only of said gases and for lowering thetemperature of certain other of said gases below the congealing pointwhereby crystals of said other gas will form in the mass of said bath ofliquid, a filter casing, iilter material within said casing, a conduitfor conducting liquid and crystals therein to the upper portion of theinterior of said casing above said filter material, a conduit forconducting said other gas through said heat exchanger for cooling themixed gas flowing therethrough and for heating said other gas, a valvefor controlling the discharge of said other gas from said heatexchanger, a conduit for conducting the liquefied portion of said gasfrom said lter to the upper portion of said column after the same haspassed through said filter.

10. An apparatus for separating oxygen and nitrogen from air comprisinga low and high pressure rectifying column, the high pressure portion ofthe column having partially rectified lliquid oxygen therein, a heatexchanger, a con- "Zduit for conducting compressed air through said heatexchanger for liquefying said air and through said partially rectifiedoxygen for evaporating the latter, a plurality of expansion valves fordischarging said liquid air and partially rectified oxygen into saidcolumn at different heights, a conduit tor conducting partially rectiedliquid oxygen. from the high 'to the low pressure side of said column, afilter within said conduit for removing solidified particles of carbondioxide from said crude oxygen, and a conduit for conducting said carbondioxide from said lter through said heat exchanger for heating thecarbon dioxide and ior cooling the compressed air entering through saidheat exchanger, and a manually operated valve for said last namedconduit through which said carbon dioxide gas is discharged.

ANDREW C. BROWN.

